1. Field of the Invention
The invention relates to an ink ejecting apparatus.
2. Description of Related Art
Today, among the non-impact printing apparatuses which are greatly expanding into the market by replacing impact type printing apparatuses, ink jet-type printing apparatuses can be cited as the simplest in theory and the easiest in terms of adding multiple gradations and color. Among these, drop-on-demand type printers that eject only ink drops used in printing are rapidly spreading because these devices have good ejection efficiency and low running costs.
As representative methods of drop-on-demand type devices, the Kyser type, disclosed in U.S. Pat. No. 3,946,398, and the thermal jet type, disclosed in U.S. Pat. No. 4,723,129, can be cited. Of these, the former is difficult to reduce in size, while the latter requires that the ink be heat-resistant since intense heat is applied to the ink. Thus each type presents an extremely difficult problem.
A new method, which simultaneously resolves the above-described defects, is the shearing mode type which uses piezoelectric ceramics and is disclosed in U.S. Pat. No. 4,879,568.
As shown in FIG. 2A, the aforementioned shearing mode type ink jet apparatus 600 is composed of a bottom wall 601, a top wall 602 and shearing mode actuator walls 603 between the bottom and top walls 601,602. These actuator walls 603 are composed of lower walls 607, which are bonded to the bottom wall 601 and are polarized in the direction indicated by arrows 611. The upper walls 605, which are bonded to the top wall 602, are polarized in the direction indicated by arrows 609. The actuator walls 603 come in pairs, and an ink flow path 613 is formed therebetween, while a space 615, narrower than the ink flow path 613, is formed between the next pair of actuator walls 603.
A nozzle plate 617 (FIG. 2B), having a nozzle 618 for each ink flow path 613 is fixed to one end of the ink flow paths 613, and electrodes 619,621 are provided as a metal layer on both side surfaces of each actuator wall 603. In addition, at the other end of each ink flow path 613, from the nozzles 618, there is a common ink chamber 626, and a manifold member 628, having filler units 627 in order to prevent the ink in the common ink chamber 626 from entering the above-described space 615, is also attached.
Each electrode 619,621 is coated with an insulation layer (not shown) in order to insulate the electrodes from the ink. Furthermore, the electrodes 619, which face the spaces 615, are connected to a ground 623, while the electrodes 621, provided in the ink flow paths 613, are connected to a silicon chip 625 which gives an actuator driving signal.
Next, the method of producing the ink jet apparatus 600 will be described. First, a piezoelectric ceramic layer which is polarized in the direction of arrow 611 is bonded to the bottom wall 601, and a piezoelectric ceramic layer which is polarized in the direction of arrow 609 is bonded to the top wall 602. The thickness of each piezoelectric ceramic layer is equal to the heights of the lower wall 607 and the upper wall 605. Next, parallel grooves are formed in the piezoelectric ceramic layers through the rotation of a diamond cutting disc, or the like, to form the lower walls 607 and upper walls 605. A metal layer is then formed on the side surfaces of the lower walls 607 through vacuum evaporation and the insulating layer is provided on top of the metal layer. Similarly, a metal layer and the insulating layer are provided on the side surfaces of the upper walls 605.
The top part of the upper walls 605 and the top part of the lower walls 607 are bound together, forming the ink flow paths 613 and the spaces 615. Next, a nozzle plate 617, in which nozzles 618 are formed, is attached to one end of the ink flow paths 613 and the spaces 615 so that the nozzles 618 correspond to the ink flow paths 613. Further, a manifold member 628, in which fillers 627 and a common ink chamber 626 are formed, is attached to the other end of the ink flow paths 613 and empty spaces 615 so that the fillers 627 correspond to the spaces 615. Additionally, the electrodes, at the mainfold member and of the ink flow paths 613 and the spaces 615, are connected to the silicon chip 625 and the ground 623 respectively.
In addition, each actuator wall 603 is capable of undergoing a piezoelectric thickness deformation in the direction which increases the volume of the ink flow paths 613, through the silicon chip 625 impressing a voltage on the electrode 621 of the ink flow path 613. For example, as shown in FIG. 3, when the voltage V is impressed on the ink flow path 613C, an electric field is created in actuator wall 603E in the direction of arrows 629,631 and in wall 603F in the direction of arrows 630,632, so that actuator walls 603E and 603F undergo a piezoelectric thickness deformation in the direction which increases the volume of the ink flow path 613C. At this time, the pressure inside the ink flow path 613C, including that near the nozzle 618, decreases. This condition is maintained for the one-way traversing time T of a pressure wave in the ink flow path 613. When this occurs, ink is supplied from the common ink chamber 626 during this interval.
The aforementioned T is the time needed for the pressure wave in the ink flow path 613 to traverse the longitudinal direction of the ink flow path 613, and is determined by the length L of the ink flow path 613 and the speed of sound "a" in the ink that is inside the ink flow path 613, that is T=L/a.
According to pressure wave propagation theory, when precisely the time T has elapsed from the impressing of the aforementioned voltage, the pressure inside the ink flow path 613 reverses and switches to a positive pressure, but in accordance with this timing, the voltage impressed on the electrode 621C of the ink flow path 613C is returned to 0. When this occurs, the actuator walls 603E,603F return to the state prior to deformation (FIG. 2A), and a pressure is applied to the ink. At this time, the pressure of the pressure wave, which has turned positive and the pressure created by the actuator walls 603E,603F returning to the pre-deformation state add together, so that a relatively high pressure is created in the area near the nozzle 618C of the ink flow path 613C, causing ink to be ejected from the nozzle 618C.
However, the ink used in this kind of ejection apparatus is such that, as shown in FIG. 4, the properties change by an extremely large amount according to the surrounding temperature with, for example one particular ink having a viscosity of around 3 mPa.multidot.s at 25.degree. C., but having over 6 mPa.multidot.s at 10.degree. C. or less, and around 2 mPa.multidot.s at 40.degree. C. In addition, the rate of change of the electromechanical coupling coefficient of the electromechanical energy conversion device used in this kind of ejection apparatus, with changes in the temperature, is on the order of 3500 to 8000 PPm/.degree. C. The rate of change is particularly large at temperatures of 20.degree. C. or less.
Because of the change in the viscosity of the ink and the changes in the electromechanical coupling coefficient of the electromechanical energy conversion device caused by changes in temperature, the ink drop ejection velocity becomes smaller at low temperatures. In the worst cases, ejection becomes impossible, creating the problem that printing quality drops.
In order to resolve this problem, the ink drop ejection velocity has been adjusted by causing the driving voltage of the ink ejection device to change in accordance with a change in the temperature around the ink ejection apparatus. However, this requires a temperature sensor and voltage conversion sensor circuit as part of the driving circuit, creating the problem that the cost of the driving circuit increases.